1. Field of the Invention
The present invention relates to an image forming apparatus, and a process cartridge which can be detachably set in the image forming apparatus.
2. Discussion of the Background
Electrophotographic image forming apparatus including a photoreceptor serving as an image bearing member, a charging roller which serves as a charging device and uniformly charges the photoreceptor, a light irradiator which irradiates the photoreceptor with imagewise light to form an electrostatic latent image, and a developing device which develops the electrostatic latent image with a toner to form a toner image on the photoreceptor are well known. The image forming apparatus also include a transfer device which transfer the toner image onto a receiving material or a transfer belt, and a cleaning device which removes toner particles remaining on the photoreceptor even after the toner image transfer operation.
As to the photoreceptor for use in the image forming apparatus, organic photoreceptors are typically used therefor because of having low cost and high productivity and being pollution free. However, organic photoreceptors have low abrasion resistance, i.e., have a relatively low abrasion resistance compared to inorganic photoreceptors. In attempting to improve the abrasion resistance, photoreceptors including a metal oxide in the outermost layer thereof have been proposed.
In addition, in attempting to fulfill a recent need for high quality images, toners having a small particle diameter and/or a spherical form have been proposed and popularly used. However, in an image forming apparatus using such a spherical toner with a small particle diameter, toner particles remaining on the photoreceptor cannot be well removed with a cleaner such as brush rollers and cleaning blades. The toner particles thus remaining on the photoreceptor without being removed are adhered to the charging roller used, thereby causing uneven charging and deteriorating image qualities of the resultant images.
Published unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 2001-109235 discloses a short-range charger in which a charging roller is arranged so as to be close to a photoreceptor while having a predetermined gap. It is described therein that by using the short-range charger, the amount of toner particles adhered to the charging roller can be reduced, resulting in prevention of occurrence of the uneven charging problem. In addition, JP-A 2001-109235 discloses to apply a DC voltage overlapped with an AC voltage to the charging roller (hereinafter this charging method is referred to as AC roller charging), to avoid uneven charging due to variation of the gap between the charging roller and the photoreceptor.
However, when the AC roller charging is performed, the amount of materials generated by high-energy discharging caused in the vicinity of the photoreceptor, such as ozone, is relatively large compared with those in the case where a contact charging roller to which a DC voltage is applied is used. The discharge-induced materials change the properties of the surface of the photoreceptors used, resulting in increase in friction coefficient of the surface of the photoreceptors. Since a photoreceptor including a metal oxide in the outermost layer thereof has a good abrasion resistance, the surface of the photoreceptor is hardly abraded, and therefore the properties of the surface are further changed, resulting remarkable increase in the friction coefficient of the photoreceptors. When the friction coefficient of the surface of photoreceptors increases, a problem in that a toner image is not well transferred to a receiving material tends to occur. Particularly, when the surface of the receiving material is smooth, this transfer problem is more frequently caused.
Image forming apparatus having a lubricant applicator which applies a lubricant to the image bearing members thereof have been proposed. The image forming apparatus have advantages such that the abraded amount of the surface of the image bearing members is reduced; the toner transfer rate is improved; formation of undesired images such as images with omissions can be prevented; and the cleanability of the image bearing members is improved.
In a case where a lubricant is supplied by a solid lubricant stick, the lubricant stick is abraded as the image forming apparatus are used and it is difficult to maintain lubricant application conditions for a long period of time.
In attempting to solve this problem, JP-As 08-234642 and 11-202569 have disclosed techniques in that the amount of toner adhered to an image bearing member is checked using a toner amount detector to determine the conditions of the lubricant adhered to the image bearing member and to maintain the surface conditions of the image bearing member.
In addition, JP-As 08-234642 and 2002-341695 have disclosed techniques in that the pressure applied to a lubricant applicator is changed or the number of rotations or rotation cycle of the lubricant applicator are changed, depending on the amount of fed papers to control the coating amount of the lubricant so as to be uniform even when the lubricant stick is abraded with time.
As mentioned above, toners having a small particle diameter have been popularly used, but such small toners have poor cleanability. By applying a lubricant to the surface of an image bearing member, the cleanability of the image bearing member can be improved. However, the conditions of the lubricant adhered to the image bearing member cannot be satisfactorily maintained.
For example, the method in which the conditions of the lubricant on the surface of an image bearing member is determined by checking the amount of the toner adhered to the image bearing member using a toner amount detector has a drawback in that the conditions determined do not necessarily represent the conditions of the entire surface of the image bearing member because the detector measures the toner amount of only a part of toner images. In recent years, compact high speed full color image forming apparatuses including tandem type image forming units have been practically used. When a toner amount detector is provided on each of the tandem type image forming units, the image forming apparatus have high cost and become large in size.
There is another method in which the lubricant application conditions are controlled depending on the image area proportion. However, when, for example, 100 copies of an image are reproduced, the rotation number of the image bearing member is largely different between a case where the copies are produced one by one and a case where the copies are continuously produced. Therefore, the conditions of the image bearing member do not necessarily correspond to the image area proportion of the produced images.
Because of these reasons, a need exists for an image forming apparatus which can stably produce high quality images without causing the charging problem and the transfer problem mentioned above even when using a photoreceptor with good abrasion resistance.